Explanation of Plate 8. 



Figure 43. — Archseocidaris wortheni, Hall ; showing 4 columns of plates in each in- 

 terambulacrum. Ventral ly the plates of the corona have been re- 

 sorbed by encroachment of the peristome. In areas A and C, plates 

 c and e are nearly full sized, but somewhat resorbed ventrally ; plates 

 d and / are less than the upper half of a hexagon, the rest of the plate 

 having been resorbed. In areas E and G a reversed condition 

 occurs, plates c and e being less than the upper half of a hexagon, 

 while plates d and / are large, with only their ventral border re- 

 sorbed. Ambulacra in the corona consist of 2 columns, a, b, of 

 low, regular plates. Plates not present in the specimen are indicated 

 by dotted lines. On the peristome dental pyramids lie opposite the 

 interambulacra. In area A one side of a pyramid is restored. In- 

 terambulacral plates of the peristome are scale-like, imbricating 

 adorally. Ambulacral plates on the peristome are somewhat irregu- 

 lar, drawn out laterally, and each has 2 pores in a horizontal plane. 

 The columns of interambulacral plates of the corona are numbered 

 1, 2, ,S, 4. For explanation of the method of arriving at these num- 

 bers compare with figure 45. Saint Louis group, Subcarboniferous, 

 Saint Louis, Missouri. Specimen in the American Museum of 

 Natural History. Magnified 2 diameters. Page 214, 



Figure 44. — The same; restoration of figure 43. Interambulacrum J is restored 

 and the plates are more regular and carried further up on the peris- 

 tome than shown in the specimen. Page 218. 



Figure 45. — The same, showing ventral border of corona, as in figure 44, and a re- 

 construction of plates, which have been resorbed. 10 ambulacral 

 plates surround the mouth, and pyramids are indicated on the 

 border of the peristome. Interambulacra are restored with the 

 same arrangement as traced in Melonites and other genera ; column 

 4 is right-handed in areas A and C and left-handed in E, G, I. 

 Introducing plates by this method, the row of plates c, d, e, f 

 above the dotted (resorption) line corresponds with the ventral 

 row of plates in figure 44. In the reconstruction a reasonable pro- 

 portionate size of plates is maintained, and they meet almost in the 

 center. Compare with plate 3, figures 8-11 ; plate 7, figures 41, 42 ; 

 plate 9, figures 54, 55; figure 1, page 164, and figures 3, 4, page 234. 

 Page 219. 



Figure 46. — Archseocidaris wortheni; plate with primary and secondary spines- 

 Saint Louis group, Subcarboniferous, Saint Louis, Missouri. Cata- 

 logue number 3028. Magnified 2 diameters. Page 217. 



Figure 47. — Cidaris Jlorigemma, Phil.; showing alternation of large and small plates 

 at ventral border of interambulacra, as in Archseocidaris. By read- 

 justment the ventral plates, like those dorsally, have complete sets 

 of spine bosses. Coral Rag, Wiltshire, England. Catalogue number 

 2005. Life size. Page 215. 



Figure 48. — Cidaris tribidoides, L.; showing alternation o^entral interambulacral 

 plates of corona, as in figures 44, 47. Ambulacra extend on to the 

 peristome as two columns of plates, as in Archseocidaris, but differ in 

 that the pores are vertically superimposed. Interambulacral plates 

 on the peristome imbricate adorally, as in Archseocidaris. Recent, 

 Panama. Catalogue number 404. Magnified 2 diameters. Page 218. 



Specimens of figures 46, 47, 48 are in the Museum of Comparative Zoology. 



253 



